Drawing the LINE: Cryptographic Analysis and Security Improvements for the LINE E2EE Protocol
2026-02-20 • Cryptography and Security
Cryptography and Security
AI summaryⓘ
The authors studied the security of the LINE version two messaging app, which is very popular in some East Asian countries. They used a special method called the Multi-Stage Key Exchange model to check how safe the app's messaging system is against hackers. They found that while LINEv2 protects messages and keys fairly well, it does not keep past messages safe if a user's device is compromised later. To fix this, the authors proposed a stronger version of the protocol that adds extra protections called forward secrecy and post-compromise security, and they tested how well their improvements work.
LINE messaging appcryptographic protocolMulti-Stage Key Exchange (MSKE)key indistinguishabilitymessage authenticationforward secrecy (FS)post-compromise security (PCS)real-world security analysisprotocol benchmarking
Authors
Benjamin Dowling, Prosanta Gope, Mehr U Nisa, Bhagya Wimalasiri
Abstract
LINE has emerged as one of the most popular communication platforms in many East Asian countries, including Thailand and Japan, with millions of active users. Therefore, it is essential to understand its security guarantees. In this work, we present the first provable security analysis of the LINE version two (LINEv2) messaging protocol, focusing on its cryptographic guarantees in a real-world setting. We capture the architecture and security of the LINE messaging protocol by modifying the Multi-Stage Key Exchange (MSKE) model, a framework for analysing cryptographic protocols under adversarial conditions. While LINEv2 achieves basic security properties such as key indistinguishability and message authentication, we highlight the lack of forward secrecy (FS) and post-compromise security (PCS). To address this, we introduce a stronger version of the LINE protocol, introducing FS and PCS to LINE, analysing and benchmarking our results.